Fibre optic distributed acoustic sensors are known. Such sensors interrogate optical fibres with optical radiation and measure changes to the radiation resulting from acoustic waves affecting the optical fibre.
U.S. Pat. No. 5,194,847 (Texas A&M Univ) describes interrogating a fibre with a repeated coherent pulse of radiation and detecting any radiation which is Raleigh backscattered within the fibre. The fibre is interrogated with a single pulse at a time and the amplitude of the backscattered radiation is analysed to detect any disturbance of the fibre by acoustic/pressure waves. This document teaches that a buried optical fibre can be used as a distributed acoustic sensor for perimeter monitoring purposes.
UK Patent Application GB 2,222,247 (Plessey) describes another distributed fibre optic sensor system in which changes in environmental parameters, such as sound waves, are sensed by transmitting pulses of light along an optical fibre. This document describes that two closely spaced pulses may be transmitted into the fibre, the first pulse having a different frequency to the second pulse. The backscatter from the pulses within the fibre can be detected and analysed at a carrier frequency equal to the frequency difference between the interrogating pulses. The signals received at a detector can be gated and processed to determine information representative of changes in environmental parameters affecting a desired section of the optical fibre.
UK patent application GB 2,442,745 (AT&T) describes distributed acoustic sensing using an optical fibre. This document again teaches use of pulse pairs, wherein the individual pulses of a pulse pair have different frequencies. The backscattered signal is analysed at a carrier frequency corresponding to the frequency difference between the pulses in the pulse pair. This document teaches applying a complex demodulation to the detected backscatter signal at the known frequency difference between the pulses in a pulse pair to provide in-phase (I) and quadrature (Q) signals for the carrier frequency. These are then converted to provide the phase and amplitude of the signal. The phase of successive samples from the same section of fibre is then monitored to determine any acoustic signals incident on that section of fibre.
This document (GB 2,442,745) teaches that the frequency difference between pulses in a pulse pair should be related to the pulse width. The example is given of pulses of 20 m width and a frequency difference between pulses in a pair of at least 5 MHz.
Whilst the technique described in GB 2,442,745 is useful, in some instances the baseband structure inherent in such a fibre optic sensor, i.e. a random but systematic pattern in the detected backscatter, can mask or destroy the carrier signal and reduce the signal to noise ratio of the sensor. This baseband structure of the system arises partly from the random distribution of the scattering sites in the optical fibre, from thermal drift etc. and thus can not be eliminated. The effect of cross over of the measurement signal and baseband noise of the system can be mitigated by using higher carrier frequencies, for instance of the orders of hundreds of MHz. However use of such a high carrier frequency would require detector sample rates in excess of hundreds of MHz. This not only would require very fast components for the interrogator unit and greatly increase the amount of processing required but a much higher detector bandwidth would also impact on the sensitivity of the detector.